Late Devonian transpressional tectonics in ... - GeoScienceWorld

Journal of the Geological Society, London, Vol. 168, 2011, pp. 441–456. doi: 10.1144/0016-76492010-046.

Late Devonian transpressional tectonics in Spitsbergen, Svalbard, and implications for basement uplift of the Sørkapp–Hornsund High S . G . B E R G H 1 * , H . D. M A H E R , J R 2 & A . B R A AT H E N 3 1 Department of Geology, University of Tromsø, N-9037 Tromsø, Norway 2 Department of Geography and Geology, University of Nebraska at Omaha, Omaha, NE 68182-0199, USA 3 Arctic Geology Department, UNIS, 9171 Longyearbyen, Norway *Corresponding author (e-mail: [email protected]) Abstract: The Late Devonian Svalbardian event relates to the final activity in the Caledonian Orogen, and affected Devonian strata in Spitsbergen by major folding, oblique thrusting and basement uplift. In southern Spitsbergen, the Devonian deformation and the complementary, presumed Mid-Carboniferous Adriabukta event deformation caused uplift of the Sørkapp–Hornsund basement high or horst. This high is fault-bounded by Devonian sandstones and a questionably aged Early or Mid-Carboniferous mudstone unit (Adriabukta Formation). The Adriabukta Formation at Hornsund occurs in the core of a major syncline, with underlying Devonian strata in the west limb, all truncated in the footwall by a steep, east-side-up oblique-reverse fault. Mid- to Late Carboniferous rifting reversed the motion and produced rift-fill deposits, and these strata overlie the deformed Devonian rocks and the Adriabukta Formation with an angular unconformity. A similar basin architecture and major syncline bounded by a reverse fault with lateral movement characterize the Svalbardian deformation in the Mimerdalen–Pyramiden area at Billefjorden farther north. Similarity also exists between a Late Devonian unit (Plantekløfta Formation) at Mimerdalen and the Adriabukta Formation at Hornsund, and we question the previous interpretation of the Adriabukta Formation as Carboniferous. Rather, we suggest that the Adriabukta and Svalbardian deformation events may have been part of the same event.

event at Hornsund created steep shear zones and tectonized mudstones and large-scale synclines (Samarinbreen Syncline) and associated thrusts in the Adriabukta Formation and Devonian units (Dallmann 1992). These deformed strata are unconformably overlain by the Moscovian Hyrnefjellet Formation and Triassic units (Fig. 2), thus constraining a Mid-Carboniferous or older depositional age for the Adriabukta Formation (Dallmann 1992). Presumed time-equivalents of the Adriabukta Formation to the west are the Hornsundneset and Sergejevfjellet Formations. A key issue is the Viseán age of the Adriabukta Formation, which is based on presumed palynology age-indicative values (Birkenmajer & Turnau 1962). Both the timing of the Adriabukta event (and its proposed temporal separation from the Svalbardian event) and the significance of the Adriabukta event in establishing the Sørkapp–Hornsund basement High are questionable, especially as contractional structures of Mid-Carboniferous age are unknown elsewhere in Spitsbergen. In this paper we use stratigraphic, structural and kinematic data to address the age and significance of the Adriabukta event. We compare presumed Early Carboniferous structures from either side of the Sørkapp– Hornsund High at Hornsund with Svalbardian event deformation in the Mimerdalen–Pyramiden area adjacent to the Billefjorden Fault Zone in northern Spitsbergen (Fig. 1). We propose that the Adriabukta event may be Late Devonian in age by pointing out the similarities of these deformations in the Hornsund and Mimerdalen–Pyramiden areas.

The Late Devonian Svalbardian event marked the termination of the Caledonian Orogeny, which affected the entire North Atlantic region. In mainland Norway, folding of Devonian basins has been associated with large-magnitude extensional deformation and the formation of extensional detachment faults and supradetachment basins (Andersen 1998; Osmundsen et al. 1998; Osmundsen & Andersen 2001). In Spitsbergen, this event was characterized by multiple and complex transpressional and transtensional deformations (Harland 1969, 1972, 1997; Gee 1972; Harland et al. 1974; Birkenmajer 1975, 1981; Lamar et al. 1986; Piepjohn et al. 2000), followed by phases of presumed Carboniferous age: (1) Early Carboniferous (Famennian–Viseán) broad basin evolution, with deposition of sandstones of the Billefjorden Group in the north and shales, siltstones and sandstones of the Adriabukta (Viseán), Hornsundneset and Sergeijevfjellet (Viseán–Namurian?) Formations in southern Spitsbergen (Cutbill & Challinor 1965; Gjelberg & Steel 1981; Steel & Worsley 1984); (2) a period of local basin inversion termed the Adriabukta event in the Mid-Carboniferous of southern Spitsbergen (Birkenmajer 1975, 1981); (3) Midto Late Carboniferous (Bashkirian–Moscovian) deposition of interfingering clastic and evaporite sediments (Hyrnefjellet and Treskelodden Formations) in narrow rift basins such as the Inner Hornsund and Billefjorden Troughs (Fig. 1; Steel & Worsley 1984). The age, kinematic nature and significance of the Adriabukta event has been debated (e.g. Birkenmajer 1964, 1975, 1984; Steel & Worsley 1984; Dallmann 1992, 1999; Dallmann et al. 1993a). Birkenmajer (1964) defined this phase as contractional based on the presence of folds and thrusts in the Early Carboniferous (Viseán) Adriabukta Formation at Hornsund, and this event was thought to have facilitated uplift of basement in the Sørkapp–Hornsund High (Figs 1 and 2). The Adriabukta

Devonian and Carboniferous basins and highs The Devonian strata of northern Spitsbergen are characterized by thick Old Red Sandstone successions (Harland 1969, 1997) formed as a result of erosion of the Caledonian mountain chain. 441

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Fig. 1. Map of Svalbard showing the main geological units, fault zones, Devonian fault blocks, Bashkirian troughs and highs, and Tertiary basins and grabens (Dallmann et al. 2002). The frames locate the study areas.

These molasse products were deposited in major north–southtrending, late- to post-orogenic foreland and intramontane basins (Andreé Land fault block) (Friend et al. 2000). Devonian strata are also exposed in the Hornsund area of southern Spitsbergen, adjacent to the Inner Hornsund Fault Zone, the eastern boundary fault zone of the Sørkapp–Hornsund High (Figs 1 and 2). These strata are folded in a major syncline (Samarinbreen Syncline) below the base-Triassic unconformity (Dallmann et al. 1993a). The Devonian stratigraphy of the Andreé Land fault block in northern Spitsbergen (Fig. 1) consists of the Wood Bay, Mimerdalen and Grey Hoek Formations (Friend 1961; Harland 1997; Dallmann 1999; Blomeier et al. 2003), whereas the Devonian sequences of Hornsund belong to the Marietoppen Formation (c. 1000 m), probably representing a thinner portion of the Wood Bay Formation (Dallmann et al. 1993a). The Late Devonian Svalbardian tectonic event was confined to the termination of the Caledonian Orogen (Harland 1969, 1972; Harland et al. 1974) and involved complex phases of transtensional and transpressional tectonics (e.g. Ziegler 1978, 1988; McCann & Dallmann 1996; Piepjohn 2000; Piepjohn et al. 2000). In the Carboniferous, basins and highs formed throughout Svalbard (e.g. Birkenmajer 1964, 1972; Steel & Worsley 1984; Worsley 1986; Maher & Welbon 1992; Braathen et al. 1995). The deposition was controlled by faults such as the palaeo-Hornsund Fault Zone offshore western Spitsbergen, the Billefjorden Fault Zone in the NE (Fig. 3) and the Inner Hornsund Fault Zone onshore farther south. These faults are thought to record tectonic instability and extensional block faulting in the Early to MidCarboniferous (e.g. Orvin 1940; Birkenmajer 1964, 1981; Harland 1969; Gjelberg & Steel 1981; Steel & Worsley 1984).

The Early Carboniferous history involved deposition of the Billefjorden Group sandstones east and west of the Billefjorden Fault Zone in the north (Cutbill & Challinor 1965; Gjelberg & Steel 1981; Steel & Worsley 1984). In southern Spitsbergen, the questionable Viseán age Adriabukta Formation (mudstones, sandstones and conglomerates), the Viseán–Namurian Hornsundneset Formation (quartz-rich sandstones) and the overlying Sergeijevfjellet Formation (shales, siltstones and sandstones) were deposited in locations to the east and west of the Sørkapp–Hornsund High (Fig. 2). The boundary between the Devonian strata and the Adriabukta Formation is locally exposed in the Hornsund area, and it is interpreted as an angular unconformity (Birkenmajer 1964). The unconformity is likely to mark a hiatus, as inferred from the close vicinity of exposures where Devonian and locally Adriabukta Formation strata lie directly on metamorphic basement (Dallmann 1992). In the Bashkirian–Moscovian, clastic sediments of the Hyrnefjellet and Treskelodden Formations were deposited in a narrow rift basin east of the Sørkapp–Hornsund High, the Inner Hornsund Trough (Steel & Worsley 1984), at the same time as other basins formed, such as the Billefjorden Trough. Worsley (1986) and Dallmann (1992) argued for the Sørkapp– Hornsund High being established when the Adriabukta Formation was deposited. This high was subsequently transgressed and partly covered in the Viseán–Namurian during deposition of the Hornsundneset and Sergeijevfjellet Formations, but re-emerged in the Bashkirian–Moscovian. The western margin of the Sørkapp–Hornsund High is defined by the Liddalen Fault, with thick Carboniferous strata (Hornsundneset and Sergeijevfjellet Formations) to the west and thin, attenuated Carboniferous strata

L AT E D E VO N I A N T R A N S P R E S S I O N I N S VA L BA R D

Fig. 2. Geological and structural map of the Sørkapp–Hornsund region, southern Spitsbergen, modified after Dallmann (1992) and Winsnes et al. (1993). The frames locate the study areas at Treskelodden and Liddalen. The straight line shows the approximate location of the cross-section in Figure 15. The distinction between Carboniferous and Tertiary structures should be noted. AB, Adriabukta; EHF, Eastern Hornsund Fault; IHF, Inner Hornsund Fault; LS, Liddalen syncline; LD, Liddalen; LF, Liddalen Fault; MF, Meranfjellet; SS, Samarinbreen Syncline; TR, Treskelodden.

on basement rocks to the east (Fig. 2). The eastern margin is inferred at Adriabukta (Fig. 2), where conglomerates in the Hyrnefjellet and Treskelodden Formations indicate a western, possibly fault-related, proximal source (Birkenmajer 1984). East of the Sørkapp–Hornsund High, strata of the Hyrnefjellet and Treskelodden Formations rest unconformably on rocks of the pre-Devonian basement. The age and uplift–subsidence history of the Sørkapp– Hornsund High is important for palaeogeographical reconstructions of other presumed Carboniferous highs on the Barents Shelf, such as the Stappen and Loppa Highs (Gabrielsen et al. 1990), and for understanding both local and regional deformation patterns. A key question is whether the Sørkapp–Hornsund High owes its origin to Late Devonian Svalbardian deformation, Early to Mid-Carboniferous deformation (i.e. Adriabukta event) and/or Carboniferous extensional basin faulting. Our structural data and kinematic reconstruction of Adriabukta event structures and comparison with the Svalbardian event structures aim to clarify this question.

Structural data We have studied structures that affected the Devonian rocks and the strata of the Adriabukta Formation near Hornsund in the Adriabukta and Liddalen areas on either side of the Sørkapp–

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Fig. 3. Map showing approximate distribution of Early Carboniferous basins (shaded areas) and bounding normal faults in Spitsbergen, after Gjelberg (1981), Steel & Worsley (1984), Maher & Welbon (1992) and Braathen et al. (1995). BFZ, Billefjorden Fault Zone; BHT, Brøggerhalvøya Trough; BT, Billefjorden Trough; IHF, Inner Hornsund Fault Zone; IHT, Inner Hornsund Trough; LFZ, Lomfjorden Fault Zone; LT, Lomfjorden Trough; PHF, Palaeo-Hornsund Fault; SHH, Sørkapp– Hornsund High; ST, St. Jonsfjorden Trough

Hornsund High (Fig. 2). In north–central Spitsbergen structural data were gathered from the Mimerdalen–Pyramiden area, where the deformation involves Late Devonian strata adjacent to the Billefjorden Fault Zone. Given the questionable age of the Adriabukta Formation (Birkenmajer & Turnau 1962), we focus on the overall geometry and kinematics of the deformation, which provide a basis to discuss correlation of the Svalbardian structures compared with those of the Adriabukta event.

Adriabukta area Structural description. The Adriabukta area at Hornsund is centred near the eastern margin of the Sørkapp–Hornsund High, within the Tertiary fold and thrust belt (Figs 2, 4 and 5). Although the investigated section shows older (Adriabukta event) deformation, the Tertiary deformation clearly affected these older structures. The outcrop section at Adriabukta (Figs 4 and 5) starts with low-grade Neoproterozoic and Cambrian basement rocks, which are overlain by steeply west-dipping and overturned Devonian Old Red Sandstone units of the Marietoppen Formation (¼ Wood Bay Formation; Birkenmajer 1964; Dallmann et

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Fig. 4. Detailed geological and structural map and interpreted cross-section of the Adriabukta area at inner Hornsund, near the eastern margin of the Sørkapp–Hornsund High. The map is modified after Dallmann (1992) and Ohta & Dallmann (1999). Location is shown in Figure 2. HyA, Hyrnefjellet anticline; EHF, Eastern Hornsund Fault; HF, Hornsundneset Formation; MSZ, Mariekammen Shear Zone. Stratigraphic abbreviations are shown in the legend of figure.

Fig. 5. Photograph mosaic with interpretation of structures in the section along the shore of Adriabukta at Inner Hornsund (see Fig. 4 for location of section). Noteworthy features are the unconformity between the Adriabukta Formation, including the Mariekammen Shear Zone and related fabrics (sketch interpretation), the overlying Hyrnefjellet Formation, which is involved in the macro-scale Tertiary Hyrnefjellet anticline and related syncline, and also, the lenses of sheared basement rocks inside the Mariekammen Shear Zone. AF, Adriabukta Formation; HyF, Hyrnefjellet Formation; TF, Treskelodden Formation; PKs, Permian Kapp Starostin Formation; TV, Triassic Vardebukta, Tvillingodden and Kistefjellet Formations; TU, Triassic, undifferentiated.

al. 1993a). Conformably above (structurally below) are mudstones and interbedded sandstones–siltstones and locally, pebble conglomerates of the Adriabukta Formation succession (Fig. 6). This variably deformed unit dips c. 508 to the west and hosts the

Mariekammen shear zone, a major bedding-parallel and semiductile deformation zone (Figs 5 and 6). The Bashkirian to Moscovian Hyrnefjellet and Treskelodden Formations unconformably overlie the Adriabukta Formation and they dip c. 358


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Fig. 6. Compiled sketches of the studied Adriabukta section showing detailed structural fabrics and geometric interpretation. Location of the section is shown in Figure 5.

east (Fig. 5). These units are followed by Permian–Cretaceous strata folded in a NNW–SSE-trending macroscopic anticline, the Hyrnefjellet anticline, which is part of the Tertiary fold and thrust belt (Fig. 5). The major Eastern Hornsund Fault is a normal fault that down-drops all the strata c. 2000 m to the ENE and may be an expression of the Tertiary reactivation of the

buried Inner Hornsund Fault Zone (Dallmann 1992; Dallmann et al. 1993b). Structures observed in the deformed Adriabukta Formation strata and Mariekammen shear zone are shown in Figures 6 and 7. Orientation data (Fig. 8) are presented after restoring the Hyrnefjellet Formation to horizontal using a 0158 E and gently

Fig. 7. Photographs of representative deformation fabrics in the Adriabukta Formation. (a) Open upright folds in siltstone and mudstone layers with related main axial planar cleavage. (Note hammer for scale.) Locality is at level 400 m in Figure 6. (b) Cataclastic to semi-ductile deformed breccias within the Mariekammen Shear Zone composed of irregular clasts embedded in a phyllitic matrix. It should be noted that one of the clasts contains a folded mudstone. Person to the lower right of the picture is shown for scale. Locality level is 750 m in Figure 6. (c) Steep cataclastic shear zones in phyllite (locality level 750 m in Fig. 6). The cliff is c. 10 m high. (d) Complex cataclastic shear zone and related asymmetrical folding within Adriabukta phyllite adjacent to the Mariekammen shear zone (locality level 440 m in Fig. 6). Person to the right is shown for scale. All photographs are viewed toward the north.

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Fig. 8. Structural and kinematic data from the deformed Adriabukta Formation at Inner Hornsund, plotted in equal-area lower hemisphere stereo nets. All data are restored to a pre-Tertiary configuration using a 0158N rotational axis and 358 angle of dip. (a) Meso-scale upright fold axes. (b) Poles to bedding of upright folded strata. (c) Contoured poles to the main cleavage. Great circle is average orientation of the main cleavage. (d) Contoured poles of movement planes (M-plane; Goldstein & Marshak 1988) on cleavage-parallel shear zones. (e) Steep cataclastic shear zones (great circles) with related drag-fold axes shown as points. (f) Moderately dipping cataclastic shear zones (great circles) with related drag-fold axes (points).

(,108) south-plunging axis and 358 dip angle (i.e. strike and dip of bedding above the unconformity). This is done to remove the effect of the Tertiary deformation. Structural data were also gathered from Meranfjellet south of the fjord and on the eastern limb of the Samarinbreen Syncline (Fig. 2). The inverted section (Fig. 6) in the west starts with a c. 350 m thick series of wellbedded sandstones, pebble conglomerates and mudstones that show only modest deformation and a cleavage parallel to bedding. These rocks grade into another 300 m thick unit characterized by interlayered mudstones and siltstones with coherent folds and bed-oblique cleavage (Fig. 7a). The bedding-parallel cleavage preserved in the western strata is composed of illite and displays no evidence of shearing, thus it is thought to have formed by compaction and pressure solution during burial. These strata are affected by asymmetric WSWverging meso-folds (Fig. 7a) displaying moderately to steeply (20–808) NNW-plunging axes (Fig. 8a and b). The folds have a

steeply dipping axial-planar cleavage that overprints bedding and the pressure solution cleavage. When restored, this second cleavage dips steeply to the ENE (Fig. 8c) and becomes the dominant, penetrative foliation of the Mariekammen shear zone (Fig. 6). Locally, cleavage-parallel shear zones or faults are abundant, and they largely obscure the earlier fabrics. Restored shear zones are generally steep, ENE-dipping and comprise dipslip striations and kinematic indicators consistent with up-to-theWSW hanging-wall movement (Fig. 8d). The c. 300 m thick Mariekammen shear zone is characterized by intensively, semi-ductile deformed meta-pelites, cataclasites and breccias (Fig. 7b). An increase in strain towards the shear zone is marked by replacement of primary bedding by networks of cross-cutting semi-ductile structures that include fold–fault populations and multiple cleavages and shear zones formed axial-planar to folds and/or subparallel to steep fold limbs (Fig 4, cross-section, and Fig. 6). Relic bedding is preserved only as


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dispersed metre-long sandstone–siltstone lenses embedded in a cataclastic matrix (Fig. 7b) and aligned parallel to the shear zone boundaries. Macro-lenses of basement Hecla Hoek rocks are also embedded along strike of the shear zone uphill (Fig. 5). Some clasts are sigmoidally shaped, laterally distorted and even folded (Fig. 7b), indicating successive reworking after folding, which is typical for progressive shear zone evolution. Furthermore, sets of narrow shear zones, locally with foliated cataclasite, are superimposed on the main foliation (Fig. 6). These later shear zones define oblique sets that produce imbricate fans, associated dragrelated folds and flower-like structures (Fig. 7c and d). When restored, one set strikes north–south and is steeply WSW-dipping (Fig. 8e), whereas the other set dips moderately ENE (Fig. 8f). Gently plunging, ENE-verging drag-folds of the main foliation into the WSW-dipping shear zones suggest up-to-the-ENE reverse motion (Fig. 8e), whereas drag-folds confined to the ENE-dipping shear zones have axes that spread along a girdle that overlaps with the shear zone, thus yielding a dominant reverse up-to-the-WSW displacement and subsidiary dextral and sinistral strike-slip components (Fig. 8f). The youngest structures that influenced the Adriabukta area are directly related to the Tertiary Hyrnefjellet anticline (Figs 4 and 5), which has an overall NW–SE-trending fold axis (Fig. 9a). Smaller-scale structures include a set of conjugate ENE- and WSW-dipping brittle normal faults that cross-cut all other structures (Fig. 6) and trend parallel to the Tertiary Eastern Hornsund Fault (Fig. 4). Striation fibres on the faults reveal normal down-to-the-SW and -NE offsets (Fig. 9b). Similar faults occur in the overlying Hyrnefjellet Formation through the Cretaceous section, suggesting that these are Tertiary in age. Notably, most slip surfaces related to the Hyrnefjellet anticline are bedding-parallel and probably formed as a result of flexural slip movement perpendicular to the macro-fold axis (Fig. 9c), which is in marked contrast to the steep bed-truncating faults in the Adriabukta area (Fig. 8e and f). Age and kinematic considerations. A pre-Moscovian age of the deformation of the Adriabukta Formation, and the Mariekammen shear zone, is evident from the angular unconformity to the overlying Hyrnefjellet Formation. A similar unconformity exists farther south between this formation and the underlying Samarinbreen Syncline (Dallmann et al. 1993a). A key observation in the Adriabukta area is that a substantial thickness of Devonian strata and conformable overlying Adriabukta Formation pebble conglomerates and mudstones overlie basement rocks in the west, whereas to the east and south the Early Carboniferous Hornsundneset and the Mid-Carboniferous Hyrnefjellet Formations are much thicker and are inferred to partly rest directly on top of basement rocks (Fig. 4, cross-section). To the south, at Samarinbreen, the Adriabukta Formation always lies between the basement and the Hornsundneset Formation. These observations suggest that a pre-Carboniferous, east-side-up fault preserved the Devonian on the west side, whereas a reversal of the fault movement occurred in the Mid-Carboniferous. Restoring the bedding–cleavages of the Adriabukta Formation and the Mariekammen shear zone produces a steep easterly dipping fault; that is, a fault with a significant reverse component. This indicates that the tilted Devonian–Adriabukta Formation section at Adriabukta and the upright meso-scale folds are a continuation of the Samarinbreen Syncline (Fig. 2), and the Mariekammen shear zone is an associated steep, ESE-dipping reverse fault (see Birkenmajer 1964, 1975). The intense folding and deformation in the Adriabukta Formation is consistent with that expected in a complex footwall syncline (see Fig. 4, cross-

Fig. 9. Structural and kinematic data for Tertiary structures in the Adriabukta and Treskelodden areas plotted in equal-area lower hemisphere stereo nets. (a) Contoured poles to bedding of folded strata in the macroscopic Hyrnefjellet anticline. (b) Slip-linear plot of mesoscopic striated faults adjacent to the Tertiary Eastern Hornsund Fault, in which the pole of the fault is a point with an arrow indicating the movement plane (Goldstein & Marshak 1988). (c) Slip-linear plot of Tertiary mesoscale normal faults in the Adriabukta Formation section (normal faults in Fig. 6) (see (b)).

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section). Furthermore, to produce the slivers of basement rocks within the shear zone the kinematic history of the Mariekammen shear zone must have been complex and multistage. Notably, the less deformed part of the Adriabukta Formation includes upright folds and axial-planar cleavages that consistently match the orientation of steep reverse faults, in addition to oblique strike-slip faults in the Mariekammen shear zone, suggesting that the folds facilitated reactivation of the shear zone. A time-progressive development is favoured from the change in deformation style; that is, major (and parasitic) folding (Samarinbreen Syncline development) and reverse faulting followed by oblique strike-slip motions (i.e. transpression), in which the strike-slip faults represent throughgoing faults that deform the main cleavage and shear zone fabric both inside and outside the Mariekammen shear zone (Fig. 4, cross-section).

Liddalen area Structural description. The western margin of the Sørkapp– Hornsund High is bounded by the Liddalen Fault (Dallmann et al. 1993a), which strikes north–south and dips steeply west and can be traced irregularly southwards through Liddalen (Fig. 2). In this area (Fig. 10a and b) the Triassic strata and underlying Lower to Mid-Carboniferous Hornsundneset and Sergeijevfjellet Formations were down-dropped to the west by c. 800 m along this fault. The Sergeijevfjellet Formation occurs close to the fault in the hanging wall and pinches out c. 2 km farther west. East of the fault, Triassic strata unconformably overlie basement rocks of the Sørkapp–Hornsund High and are monoclinally flexed down c. 500 m to the west across the Liddalen Fault. Farther west, Triassic strata conformably overlie the Carboniferous strata (Fig. 10). Notably, the Hornsundneset Formation is affected by a macroscopic NNW–SSE-trending flexure (Liddalen syncline) in the footwall of the Liddalen Fault. The eastern limb of this syncline is unconformably overlain by Triassic strata at Kovalevskifjellet, and the western limb is down-dropped by the Liddalen Fault (Fig. 10b). Similar flexures occur in the Hornsundneset Formation at Kulmstranda (Fig. 10). Poles to bedding of the flexed strata yield an irregular girdle indicating a NW– SE-trending axis (Fig. 11a). Occurrence of meso-scale striated faults in the flexed strata adjacent to the Liddalen Fault yields a group of dominantly dip-slip normal faults parallel to the Liddalen Fault and a subordinate group of strike-slip faults (Fig. 11b). The Liddalen area also experienced a strong Tertiary overprint (Dallmann 1992; Dallmann et al. 1993a; Winsnes et al. 1993). The Tertiary fold and thrust structures, however, are distinguished by their detachment fold style linked to thrusts at a low angle to bedding; for example, as observed in Sergeijevfjellet and Lidfjellet (Fig. 10a). These fold–thrust structures project into the Liddalen Fault and are either offset by the fault or ramp upward to the east. Mesoscopic strike-slip faults of assumed Tertiary age strike on average NE–SW (Fig. 11c). They appear in Triassic rocks along the trace of the Liddalen Fault. Age and kinematic considerations. The Liddalen Fault is interpreted as a predominantly Carboniferous normal fault with a later, post-Triassic (Tertiary) history. The fault is roughly parallel to the basement structural grain, and thus probably represents basement reactivation. As the Lower Carboniferous Hornsundneset Formation is missing in the footwall east of Kovalevskifjellet, probably owing to eastward thinning, pinch-out and/or erosional truncation by the overlying Triassic strata (Dallmann et al. 1993a), a pre-Triassic fault must have existed at this site. The

Fig. 10. (a) Detailed geological and structural map of the Liddalen area along the western margin of the Sørkapp-Hornsund High. See Figure 2 for location.

presence of a normal fault is supported by the major downward flexure (syncline) of the Triassic and older strata and a preTriassic structural relief across the fault, which is close to 500 m (Fig. 10b). It is interesting to note in cross-sections that restoring the Triassic to continuity leaves a hanging-wall-down component with a net extensional offset of, locally c. 500 m.

Mimerdalen–Pyramiden area Structural description. The Mimerdalen–Pyramiden area constitutes the southwestern part of the Devonian Andreé Land fault block, which is bounded to the east by the Billefjorden Fault Zone (Fig. 1) (Orvin 1940). The eastern part of the Mimerdalen–Pyramiden area is bounded by the Balliolbreen Fault (Harland et al. 1974), which strikes north–south and dips steeply east (Fig. 12). This fault represents the westernmost segment of the Billefjorden Fault Zone (Harland et al. 1974) and separates Carboniferous synrift deposits of the eastern hanging wall from Devonian units in the western footwall (Fig. 12c). Flat-lying Late Carboniferous and Permian strata unconformably overlie the Devonian and Carboniferous strata. The overall geometry encountered in the Devonian domain, as seen in the map and cross-


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Fig. 10. (b) Interpreted cross-sections of the Liddalen area. Cross-section lines are outlined in Figure 10a.

section (Fig. 12), is that of a footwall syncline below a major reverse fault complex. The entire Devonian succession is folded into this syncline, the Mimerelva syncline, and its steeply westdipping back-limb is truncated by the Munindalen thrust. The upper hanging-wall thrust segment carries the Wood Bay Formation (Friend & Moody-Stuart 1972). The Wood Bay Formation consists of a sequence of maroon and green shales and medium-grained quartz-rich sandstones of unknown thickness. Below the thrust, the Mimerdalen Subgroup (Dallmann et al. 2004) is made up of reddish, light grey sandstones and mudstones, and thick series of grey–green, fluviatile arkoses and grits belonging to the Tordalen and Planteryggen Formations (Fig. 12b). This succession is topped by a very distinctive suite of dark shales and associated coarse conglomerates, sandstones, siltstones and mudstones that make up the Plantekløfta Formation (Piepjohn et al. 2000). The clasts of the Plantekløfta Formation conglomerates are chiefly wellrounded Devonian sandstones, intermixed with fragile shale intraclasts, a composition that is differs from that of the underlying conglomerates, which in addition, host metamorphic basement clasts. In general, the thick conglomeratic layers of the Mimerdalen Subgroup and Plantekløfta Formation appear to be debris-flow and braided river deposits of relatively proximal origin, and most of them show clear signs of fluvial reworking. The strata of the Plantekløfta Formation are notably less well lithified than underlying Devonian sandstones of the Mimerdalen Subgroup. Based on palynology the Plantekløfta Formation is interpreted as Late Famennian in age or possibly earliest Carboniferous (Piepjohn et al. 2000). The Plantekløfta Formation is well exposed in the southeastern

slopes of Odinfjellet (Fig. 12a), where it is interpreted to truncate the underlying sandstones of the Mimerdalen Subgroup. Piepjohn (2000) mapped this contact as a thrust, but we consider it to be an angular unconformity as there is no stratigraphic repetition or omission across the contact that necessitates a fault. In addition, there is a lack of fault rock development or small-scale structures in close proximity to the contact, and the overlying strata have a character consistent with an origin as a basal sequence above an unconformity. There is a distinctive difference in tectonic imprint across the Munindalen thrust segment (Fig. 12c). The Wood Bay Formation of the hanging wall is tightly folded and contains two spaced tectonic cleavages. The cleavages are coated with mica, consistent with lower greenschist-facies metamorphism. In contrast, the footwall rocks show pressure solution cleavage in shales, along axial planes and within thrusts or shear zones, reflecting lower strain at diagenetic conditions. The upper, eastern segment of the Munindalen thrust has the most significant offset, of at least 1.5 km, but probably much more offset due to the abovementioned jump in metamorphic grade. This fault has a dip of c. 608 or more, and is better constrained as a high-angle reverse fault than a thrust; that is, similar to the Balliolbreen Fault and other high-angle faults in the Billefjorden Fault Zone that involved Devonian rocks. A hanging-wall cleavage–bedding flat and a consistent footwall oblique cutoff are seen along the fault. The distinctly oblique-truncating character of the Munindalen thrust into the Mimerelva syncline suggests that it is out-ofsequence with respect to the underlying structure. Bedding orientations (Fig. 13a–d) confirm an out-of-sequence character of the Munindalen thrust. Beneath the thrust, the

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occurred as a consequence of renewed uplift and contractional or strike-slip motions along, for example, the Billefjorden Fault Zone to the east, gradually producing the Mimerdalen syncline as part of a Svalbardian fold–thrust belt system (e.g. Harland 1969; Harland et al. 1974; Lamar et al. 1986; McCann & Dallmann 1996). Discontinuous multistage folding and out-ofsequence thrusting may have caused deposition of, for example, the Late Famennian Plantekløfta Formation in a foreland basin and/or in the eroded core of the Mimerdalen syncline during waning stages of the deformation. The age of the Plantekløfta Formation thus constrains a long-term depositional period for the strata involved in the Mimerdalen syncline prior to Carboniferous normal faulting along the Balliolbreen–Billefjorden Fault Zone and synrift deposition in the Billefjorden Trough further east (Harland et al. 1974; McCann & Dallmann 1996). The final events included deposition of Late Carboniferous and younger successions and a mild Tertiary contractional reactivation (McCann & Dallmann 1996; Maher et al. 2003).

Discussion

Fig. 11. Structural and kinematic data from Liddalen. (a) Poles to bedding of major syncline in the Hornsundneset Formation strata in the footwall to the Liddalen Fault. (b) Slip-linear plot (Goldstein & Marshak 1988) of meso-scale faults in the Hornsundneset Formation adjacent to the syncline. (c) Poles to bedding of Tertiary folded Triassic strata. (d) Contoured poles to bedding of Tertiary folds in Triassic strata. (e) Slip-linear plot of meso-scale faults in Triassic rocks.

Mimerelva syncline (sub-area B) has a well-constrained shallowly NE-plunging axis. In contrast, in sub-areas A and C, which are influenced by the thrust, the folding is non-cylindrical, indicating a two-phase deformation. Slip-linear data (Fig. 13e and f) for reverse faults show a significant spread in strike directions. However, folded thrusts in the Mimerelva syncline suggest early west-directed movements, whereas data from the nearby Munindalen thrust are dominated by NNW motion(s), supporting the two-stage chronology. This is consistent with the Munindalen thrust having an earlier significant strike-slip component in this area and also is consistent with the requirement for Devonian sinistral strike-slip motion along the Billefjorden Fault Zone (McCann & Dallmann 1996). Age and kinematic considerations. The field observations presented above indicate a complex evolutionary history of the Devonian strata in the Mimerdalen–Pyramiden area. The arkosic sandstone and conglomerates of the Tordalen Formation above the Wood Bay Formation were deposited in a sustained tectonic environment west of an uplifted(?) basement in the east, and followed by deposition of the Planteryggen Formation and quartz arkoses during waning tectonism in the Early or Mid-Devonian (see Blomeier et al. 2003). A resumption of deposition of coarser sandstone and quartzites of the Planteryggen Formation probably

The age and significance of the Adriabukta deformation relies heavily on a palynological Early Carboniferous (Viseán) age of the Adriabukta Formation (Birkenmajer & Turnau 1962), which is critical for the age determination of the Adriabukta event and thus for inference of a possible correlation of Svalbardian and Adriabukta event structures (Fig. 14). New palynology data by Krajewski & Stempien´-Sałek (2003) revised the Siedlecki & Turnau (1964) age determination for the overlying Sergeivfjellet Formation as Viseán instead of Namurian, thus suggesting that the Adriabukta Formation also may be older than previously thought. The idea that the Adriabukta Formation is Late Devonian in age (W. Dallmann, pers. comm. 2009) would indicate a possible Late Devonian age of the deformation, based on similarities of the involved stratigraphy, structural styles and kinematics (Fig. 14). Below we discuss these similarities and their implications for tectonic evolution, the control of basement fabric, and the significance of uplift of the Sørkapp–Hornsund High.

Devonian and Adriabukta Formation stratigraphy The Devonian strata in the Mimerdalen–Pyramiden area include continental alluvial or fluvial sandstones of the Wood Bay Formation and overlying Mimerdalen Subgroup. The lower two formations of the Mimerdalen Subgroup probably formed in a progressively evolving, sustained basin caused by basement uplift to the east (Fig. 14a). The Late Famennian Plantekløfta Formation marked a hiatus during the uplift, and was probably deposited in a restricted, vanishing foreland basin in front of a Late Devonian fold–thrust system (e.g. Mimerdalen syncline and Munindalen thrust). In the Hornsund area, a much thinner portion of the Wood Bay Formation is preserved, and the overlying Adriabukta Formation was deposited in stratigraphic conformity. Both units are involved in the Samarinbreen Syncline (Fig. 14b). The simplest explanation would be that the Adriabukta Formation is Late Devonian in age (see Birkenmajer 1964) and correlated with the Mimerdalen Subgroup. Lithological and stratigraphic similarities of the Adriabukta and Plantekløfta Formations do exist, in that both include immature, dark silty mudstones and sandstones, locally with very thick basal conglomerates consistent with a hiatus followed by proximal deposition in narrow basins fixed by the fault-bounded Devonian strata and adjacent basement blocks (Fig. 15a). In southern Spitsbergen


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Fig. 12. (a) Structural map of the Mimerdalen–Pyramiden area with (b) schematic stratigraphic description outlining formal units, and (c) interpreted cross-section. Location of the area is shown in Figure 1. The map is modified after Piepjohn et al. (2000) and Dallmann et al. (2004).

such basins may have existed along the eastern margin of what was to be the Sørkapp–Hornsund High and beneath the precursory site of the Samarinbreen Syncline (Fig. 1), and possibly also in other basins now located offshore, south and east of Svalbard (Eiken 1994; Grogan et al. 1999; Bergh & Grogan 2003). However, a Late Famennian age is in conflict with the previous Viseán age for the Adriabukta Formation based on microspore flora (Birkenmajer & Turnau 1962); thus more sedimentological and palynomorph documentation is required as a basis for re-dating of the Adriabukta Formation.

Svalbardian event The most striking similarities of the structures at Hornsund to those of the Mimerdalen–Pyramiden area include the following (Figs 12–14): (1) an early, steep east-dipping reverse fault that caused basement uplift to the east (Inner Hornsund Fault Zone and Billefjorden Fault Zone); (2) a major west-vergent syncline (Samarinbreen compared with Mimerelva synclines) and associated thrusts (Mariekammen shear zone and Munindalen thrust); (3) deformation style (i.e. upright to oblique-plunging mesoscopic folds and axial-planar cleavage); (4) late-stage thrusting and shear zones with a marked sinistral-reverse movement; (5) subsequent reversal of the movement on major faults (Inner Hornsund Fault Zone and Billefjorden Fault Zone) and formation of extensional Early to Mid-Carboniferous rift basin deposits (i.e. Hornsundneset–Sergeijevfjellet and Hyrnefjellet–Treskelodden

Formations compared with Billefjorden Group); (6) Tertiary contractional reactivation. The evidence for a pre-Carboniferous east-side-up reverse fault is convincing both at Hornsund and in the Mimerdalen–Pyramiden area. During this contractional phase, the Adriabukta Formation strata could have been deposited in a foreland basin contemporaneous with initiation of west-vergent major folding of the Devonian strata (Fig. 15b and c). This, however, requires that the Adriabukta Formation is Late Famennian in age and part of the Devonian succession. In the restored pre-Permian scenario for Hornsund (Fig. 14b) the Mariekammen shear zone displays a moderate ESE dip and may represent a subsidiary basement-seated fault linked to the Inner Hornsund Fault Zone farther east (Fig. 15e). The structural associations almost fully mimic those of the Billefjorden Fault Zone–Balliolbreen Fault and associated Munindalen thrusting of the Wood Bay Formation strata over the tilted Mimerdalen Subgroup in the footwall syncline (Fig. 14a). Furthermore, the internal kinematic characters and chronology of mesofolds, axial-planar cleavage and oblique-slip structures in the Mariekammen shear zone suggest multistage reactivation and probable partitioned transpression (see Tavarnelli et al. 2004). By analogy, the Svalbardian deformation in the Mimerdalen–Pyramiden area was multistage and characterized by folding and twophase cleavage development, sinistral-reverse faulting and oblique-truncating, out-of-sequence thrusting (Fig. 13). In such a model the Mariekammen shear zone and/or Inner Hornsund Fault Zone may have initiated as steep, WSW-directed Late Devonian reverse faults (Fig. 15b) that changed into a

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Fig. 13. Structural and kinematic data from the deformed Devonian succession of the Mimerdalen–Pyramiden area. Major fold and fault structures are divided into subdomains. (a) Poles to folded bedding of sub-domain A indicating non-cylindrical (polyphase) folding with a poorly constrained fold axis plunging NNE. (b) Poles to bedding of sub-domain B, showing a general horizontal NE–SW fold axis. The slip-linear arrows (method of Goldstein & Marshak 1988) are based on slickenside data and suggest flexural slip movement perpendicular to the fold axis. (c) Poles to bedding of sub-domain C, indicating non-cylindrical (polyphase) folding with a poorly constrained fold axis plunging gently north. (d) Plot of cleavage and fold-axis intersection lineation for subdomains A–D. (e) Slip-linear plot of slickenside data from sub-domains A and C. (f) Slip-linear plot of slickenside data from sub-domain D.

dominantly strike-slip modified structure as a result of obliquecontractional reactivation (Fig. 15c). The most intensive lateral shearing occurred along the eastern limb of the Samarinbreen Syncline, and thus may have been localized to the faulted margins of the Sørkapp–Hornsund High to the west. The orientation of the faulted margins may have been controlled by favourably oriented and steep Caledonian basement fabrics, which have overall north–south strikes at Liddalen and NNW– SSE strikes at Inner Hornsund (see Winsnes et al. 1993). These basement margins were then reactivated in the Late Devonian as combined reverse (uplift) and strike-slip faults (Fig. 15c). Alongstrike transport from a bend involving the basement may explain the incorporation of basement lenses within the Mariekammen shear zone. An irregular shape of a hypothesized Late Devonian Sørkapp–Hornsund High may be inferred from an along-strike discontinuity in thickness and width of the Adriabukta Formation strata and the Samarinbreen Syncline, from c. 8 km width in central portions of Hornsund to nearly pinched-out northwards over a cross-strike width of some 20 km (Fig. 2). The major folding and oblique sinistral-reverse faults at Mimerdalen–Pyramiden near the Billefjorden Fault Zone (Fig. 13) and the observed oblique-slip signature of the Mariekammen

shear zone indicate a possible genetic link to the Late Devonian sinistral strike-slip faulting that is proposed to have moved major portions of western Svalbard northwards with respect to central and eastern Svalbard (e.g. Harland 1969, 1997; Birkenmajer 1975; Ziegler 1978, 1988; McCann & Dallmann 1996; Piepjohn 2000; Piepjohn et al. 2000). In this context, the transcurrent motion may provide a link to the enigmatic folding and extensional reactivation of Devonian basins in southwestern Norway (see Osmundsen et al. 1998). The temporal and spatial switch in kinematics from contraction and fold–thrust belt evolution to strike-slip motions and transtension could have been accomplished by a transient local and/or anomalous locking bend associated with the Svalbardian mega-shear system (see Ziegler 1988). Consequently, the uplift of the Sørkapp–Hornsund High could have been initiated by lateral macroscopic bending of irregular-shaped Devonian faults during the Svalbardian event. The pinch-out along strike supports a slight obliquity or bending between the internal structures of the Sørkapp–Hornsund High linking the boundaries to a potential larger deformation zone offshore to the west (e.g. the palaeo-Hornsund Fault; Fig. 2), thus suggesting a transpressional origin of the Sørkapp–Hornsund High (see Steel & Worsley 1984). This obliquity has the basin


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during the Famennian–Viseán (Cutbill & Challinor 1965; Harland et al. 1974; Gjelberg & Steel 1981; Steel & Worsley 1984; McCann & Dallmann 1996). In southern Spitsbergen, the Viseán–Namurian Hornsundneset and Sergeijevfjellet Forma-

Fig. 14. Sketches and comparison of the structural architectures from (a) the Mimerdalen–Pyramiden area near the Billefjorden Fault Zone (current structure unaffected by Tertiary deformation), and (b) Inner Hornsund area showing the position of the inner Hornsund Fault and subsidiary Mariekammen Shear Zone (restored pre-Permian scenario). Abbreviations are as in Figure 12.

counterclockwise from the upside fault margin to the east, which is similar to what we see with the Mimerelva syncline. Both are consistent with a sinistral component. In this context, both the Adriabukta Formation and the Plantekløfta Formation would represent small and discontinuous Fammenian foreland basins in the footwall of major reverse and sinistral strike-slip faults.

Carboniferous event The Early to Mid-Carboniferous period in north–central Spitsbergen involved normal faulting and rift deposition of the Billefjorden Group sandstones east of the Billefjorden Fault Zone

Fig. 15. Tectonic evolution along a schematic west–east cross-section of the Sørkapp–Hornsund region for the Devonian, Carboniferous and Tertiary time periods and the implications for the Sørkapp–Hornsund High, its margins and related basins. (a) Deposition of Devonian Old Red Sandstone in a basin east of the precursory Sørkapp–Hornsund High and west of the precursory Inner Hornsund Fault Zone, and possibly also in basins farther east. (b) Late Devonian east-side-up reverse faulting (Inner Hornsund Fault Zone?) and deposition of the Adriabukta Formation strata in one or more foreland basins contemporaneous with initiation of west-directed major folding of the Devonian strata. (c) Presumed Late Svalbardian contraction including major synclinal folding (Samarinbreen Syncline), west-directed reverse faulting and transpressional deformation of the Adriabukta Formation (Mariekammen Shear Zone). The last stage of the deformation was characterized by sinistral transpressional faulting and thrust reactivation along the eastern margin of the presumed Sørkapp–Hornsund High. (d) Early Carboniferous mild sag basin formation and deposition of the Hornsundneset Formation on either side of the Sørkapp–Hornsund High and/or as a continuous platform cover sequence. (e) Late Carboniferous (Bashkirian–Moscovian) crustal extension– rifting, erosion and infilling of the Inner Hornsund Trough by the Hyrnefjellet and Treskelodden Formations east of the Sørkapp–Hornsund High. It should be noted that the Hyrnefjellet Formation was deposited conformably on top of the Hornsundneset Formation along the reactivated eastern limb of the Samarinbreen Syncline. On the west side of the Sørkapp–Hornsund High, the Liddalen Fault was reactivated and a downward flexure fold established in the hanging wall of the Hornsundneset Formation strata. (f) Tertiary contraction and fold–thrust belt development involving thrust detachments or imbricates that propagated eastward from Liddalen across the Sørkapp–Hornsund High. The east-verging Hyrnefjellet anticline and related macro-scale folds developed to the east of the basement high. Basement involvement may have also modified the Samarinbreen Syncline to become east-verging. Subsequent extensional reactivation occurred along the Inner Hornsund Fault Zone to the east and the Liddalen Fault in the west. SHH, Sørkapp– Hornsund High; D, Devonian; HF, Hornsundneset Formation; SF, Sergeijevfjellet Formation; AF, Adriabukta Formation; HyF, Hyrnefjellet Formation; TF, Treskelodden Formation; LF, Liddalen Fault; LS, Liddalen Syncline; MSZ, Mariekammen Shear Zone; SS, Samarinbreen Syncline; HyA, Hyrnefjellet anticline; EHF, Eastern Hornsund Fault; IHF, Inner Hornsund Fault Zone. Large open arrows mark either extension or contraction, and half-arrows show sense of displacement on faults. Note that in the Tertiary, contraction was followed by extension.

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tions are assigned as temporal equivalents of the Billefjorden Group (Dallmann et al. 1993a), deposited in basins to the east and west of the Sørkapp–Hornsund High (Fig. 2). However, the clean and distinct mature, fluvial quartz-rich sandstones that characterize the Hornsundneset Formation contrast markedly with the immature mudstones and conglomerates of the underlying Adriabukta Formation (see Birkenmajer 1964). The bedding-parallel cleavage in the Adriabukta Formation at Inner Hornsund is suggestive of some overburden that was locally removed prior to deposition of the Hyrnefjellet and Treskelodden Formations. Hence, a pre-Hornsundneset Formation basin must have existed (Fig. 15d). The basin character (c. 10 km wide) and an inferred cumulative thickness of c. 1750 m for the Adriabukta Formation (Dallmann 1999) are similar to those of other Carboniferous basins in Svalbard (Fig. 3); that is, the quartz-rich and mature sediments seem to reflect a stable shelf setting rather than a rift facies (Dallmann 1999). Rifting is typically used to explain Early Carboniferous basin formation (e.g. Worsley 1986), but in contrast to the later Bashkirian narrow basins (e.g. the Billefjorden Trough), there is no direct evidence of active faults of this age in Spitsbergen. However, the sedimentology and lateral continuity of the Hornsundneset Formation on both sides of the Sørkapp–Hornsund High indicate that this unit represents an early platform cover sequence deposited in a wider basin as a tabular unit onlapping the earlier sag (Fig. 15d). Broad shallow basins could, alternatively, be ascribed to the final episodes of Svalbardian deformation, with local thickening and thermal perturbation, adjusted by mild subsidence along major faults. The Sørkapp–Hornsund High may also have been established in the Late Devonian, between the Samarinbreen Syncline and the Liddalen Fault (Fig. 15c), but clearly developed its strongest expression as a high during the long-term Carboniferous rifting event (Fig. 15d and e) that rejuvenated the earlier Devonian structural grains. The abrupt thickness increase of the Hornsundneset Formation (Fig. 15d) near the base of the Triassic and the downward flexure in the Hornsundneset Formation strata above the Lidalen Fault (Fig. 15e) suggest that the Liddalen Fault was established by Early or Mid-Carboniferous rifting and normal faulting along the western margin of the Sørkapp–Hornsund High (Fig. 15e) (see Birkenmajer 1964), which had been transgressed and partly covered in the Namurian. The high probably existed during deposition of the Sergeivfjellet Formation as constrained by the pinch-out of the formation to the west (Fig. 15d and e). It is also interesting to note that after restoration of the Triassic, there is still normal fault relief across the Liddalen Fault. During the Late Carboniferous (Bashkirian–Moscovian) clastic sediments of the Hyrnefjellet and Treskelodden Formations were deposited in the Inner Hornsund Trough east of the Sørkapp–Hornsund High (Fig. 3; Steel & Worsley 1984). This rifting reversed the motion on bordering Late Devonian reverse faults (i.e. Inner Hornsund Fault) and the Hyrnefjellet and Treskelodden Formations were deposited on top of the Samarinbreen Syncline (Fig. 15d and e). The Mid-Carboniferous geometry of the Sørkapp–Hornsund High, with a well-defined eastern fault margin and the Inner Hornsund trough, and a western margin with onlaps, flexures and normal faults, is suggestive of a footwall uplift relative to an east-facing halfgraben. The Sørkapp–Hornsund High persisted as a topographic high until the Permian, when it was covered as part of a Triassic stable shelf with deltaic deposits (Fig. 15e). Importantly, there is no evidence of any post-Hornsundneset, pre-Treskelodden Formation contractional folding that could

verify the Adriabukta deformation event, because no angular unconformity has been observed between the Hornsundneset and the overlying Hyrnefjellet Formations. Instead, as argued above, the overall structural pattern makes it more probable that the Adriabukta event happened prior to the deposition of the Hornsundneset Formation (Figs 6 and 7).

Tertiary event During Palaeogene opening of the North Atlantic and Arctic Oceans (see Ziegler 1988) a transform–transpressive fold and thrust belt developed along the western margin of Spitsbergen (Harland 1969; Birkenmajer 1972; Lowell 1972; Dallmann et al. 1993b). The Tertiary fold and thrust belt in southern Spitsbergen (Fig. 15f) created layer-parallel detachments at Liddalen, basement-involved thrusts within the Sørkapp–Hornsund High and major fold zones in the Mesozoic sequences (i.e. Hyrnefjellet anticline) that overprinted the Samarinbreen Syncline (Dallmann 1992; Winsnes et al. 1993; Ohta & Dallmann 1999). Transcurrent and oblique-normal faults and basins developed late in the succession of Tertiary events (Dallmann 1992; Bergh & Grogan 2003). All these effects may have influenced the present attitude and geometry of the Sørkapp–Hornsund High. Notably, the primary zone of Tertiary folding and thrusting dies out to the south (Fig. 1), and the Sørkapp–Hornsund High plunges in this direction. It continues in a step-over zone on the western side (Dallmann 1992), which is consistent with a step or bend in the Tertiary fold–thrust belt as it encounters the Adriabukta deformation zone. It is possible that, during a change from transpression to transtension in the step-over zones (e.g. Braathen et al. 1999), the Liddalen Fault and Inner Hornsund Fault Zone were reactivated. This may suggest that the Carboniferous and older structures controlled the location of Tertiary thrusts and normal faults and thus may have caused re-inversion of the Sørkapp– Hornsund High.

Conclusions (1) The Svalbardian deformation evident along the Billefjorden Fault Zone and at Hornsund is strikingly similar in orientation, kinematics and structural style to that of the studied Adriabukta area. Both include components of contraction that formed a major syncline (Samarinbreen and Mimerelva synclines) and narrow foreland basin deposits (Plantekløfta and Adriabukta Formations) in front of a major high-angle fault with an eastside-up reverse component and geometries consistent with sinistral transpression. Associated major upright synclinal folding, reverse or transpressional faults (Mariekammen and Munindalen thrusts) and cleavage were located along the hinge or steep eastern limb of these synclines. A pre-Mid-Carboniferous eastside-up reverse fault at the eastern margin of the Sørkapp– Hornsund High is supported by the thick Devonian strata and conformable Adriabukta Formation strata on top of basement rocks along the Samarinbreen Syncline, whereas to the west the Hornsundneset Formation is thicker and rests directly on the basement rocks. (2) There is some stratigraphic and lithological similarity between the youngest Plantekløfta Formation of the Late Devonian strata in the Andreé Land fault block adjacent to the Billefjorden Fault Zone and the presumed Early Carboniferous Adriabukta Formation at Hornsund. In addition, Adriabukta Formation strata are lithologically dissimilar to Viseán age Billefjorden strata elsewhere on Spitsbergen. The simplest explanation of this is that the previously inferred Viseán age of


the Adriabukta Formation based on palynology is uncertain. This hypothesis, however, must await re-dating and better sedimentological documentation. (3) The Sørkapp–Hornsund High may have initiated during the multistage Svalbardian contractional and transpressional event, with its eastern margin defined by the late-phase sinistral transpression, and thus experienced Early to Mid-Carboniferous inversion. In this respect the Sørkapp–Hornsund High is similar to the Andree Land fault block west of the Billefjorden Fault Zone. (4) In the Mid-Carboniferous (Viseán–Serpukhovian?) the kinematic regime switched to dominantly crustal extension, perhaps as a result of changing plate boundary conditions causing a change in the internal plate stress. In southern Spitsbergen, the Namurian Hornsundneset and overlying Sergeijevfjellet Formations were deposited in flanking basins onlapping the Sørkapp–Hornsund High. (5) Along the eastern margin of the Sørkapp–Hornsund High, near the eastern limb of the Samarinbreen Syncline, a narrow rift basin subsequently formed (Inner Hornsund Trough), which reversed the motion on bordering Late Devonian reverse faults (i.e. Inner Hornsund Fault Zone), and the Hyrnefjellet and Treskelodden Formations were deposited on top of the truncated macro-fold limbs of the Samarinbreen Syncline. The Sørkapp– Hornsund High persisted as a topographic high until Triassic times, when the entire area was eroded and covered by Triassic shelf and deltaic sediments, and finally Tertiary contractional reinversion and subsequent extension. Saga Petroleum, Norwegian Polar Institute, University of Tromsø, University of Nebraska at Omaha and the Geological Survey of Norway are acknowledged for financial and logistical support to field work for this project. The authors would like to thank W.K. Dallmann and P.T. Osmundsen for their helpful and constructive reviews of the manuscript.

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Received 29 March 2010; revised typescript accepted 11 October 2010. Scientific editing by Ian Alsop.